I have read many articles, but I don't know why often the verb "discuss" is followed by the conjunction "whether," but not "if". Do you confirm that this verb is an exception and after that we have to use always "whether"?
In other words as an example I believe that:

Interesting. I have not looked at this topic like this @DamkerngT. Hope natives confirm this idea over this specific example of mine. :) Anyway still I think after some certain verbs (for instance, discuss,) it is not possible or rather grammatical to use "if".
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A-friendMay 1 '14 at 10:14

1

It's possible, but not common; you could say "we were discussing if it would be okay to leave early", for example.
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HellionMay 1 '14 at 17:19

@Hellion But that's an ordinary finite-verb clause, not an infinitive.
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StoneyBMay 1 '14 at 17:48

@StoneyB, yes, but my point is that the mere presence of "if" does not mean you cannot use it after "discuss", as A-friend was wondering in the comment above.
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HellionMay 1 '14 at 17:52

The sentence "We were discussing if it's okay to leave early" is probably a poor example of this, because if it's okay to leave early is a projected clause; the reported speech means that you can use it in that way.
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jimsugMay 17 '14 at 16:21